Rotating electrical machines are formed by a rotor and a stator, one of these members generating a rotating electrical field in such a way as to drive the other member in rotation.
In practice, the stator is generally formed by a plurality of phase windings which are distributed around its periphery and which are supplied in succession in such a way as to create the rotating magnetic field.
In this context, the rotor carries a winding through which a current of an intensity greater than a given value has to flow if the winding is to be sufficiently magnetised to be driven by the rotating magnetic field.
Because of this, the general procedure, before the rotating magnetic field is generated by the stator, is to perform a pre-magnetisation step in which a voltage is applied to the terminals of the rotor winding, which gives rise to an increasing current in the latter.
Patent application FR 2 875 556 for example describes a solution for anticipating, as much as is possible, the need for the electrical machine to be started and consequently the need for a pre-magnetisation step to be carried out. In this document, the starting of the electrical machine by the exciting of the stator is carried out with a predetermined delay (150 ms in the example given) relative to the beginning of the pre-magnetisation.
This delay time is calculated to ensure that the rotor is adequately magnetised in the different electrical conditions to which the system may be subject and the pre-magnetisation time is thus not optimum, especially in systems where use is made of a store of energy which results in a voltage liable to vary in quite wide proportions, but also because of the uncertainty which there is about electrical values in any type of system.